Electromagnetic windows are usually designed to cover and protect a radiation source while maintaining high transmission of the radiation generated thereby, and are typically based on one or more planar or shaped dielectric layers. Electromagnetic windows can be divided into two groups: all-dielectric and metal-dielectric.
The all-dielectric windows are built from either a single dielectric layer or multiple dielectric layers, designed to maximize the transmission at specific frequency bands. U.S. Pat. No. 5,958,557 discloses an electromagnetic window having a single layer of half-wavelength thickness. This window is characterized by a rather narrow frequency-band due to its resonant character. At optical frequencies, the use of even thicker windows is proposed. These are multi-layer structures with various half-wavelength and quarter-wavelength sequences designed to filter the radiation and allow the transmission of only a specific frequency band.
In systems operating with radio and microwave frequencies, the use of an electrically thin window (of a thickness significantly smaller than a wavelength to be transmitted) enables to provide broadband low-loss transmission. This is achieved by one or more rigid-foam or honeycomb cores with two or more dielectric skins. This is disclosed, for example in U.S. Pat. Nos. 3,780,374 and 4,358,772.
Window-devices utilizing a metal-dielectric combination are of two types, In the first type, the added metal structure is aimed at improving or augmenting the window performance. U.S. Pat. No. 4,467,330 discloses the use of an inductive screen incorporated inside a solid dielectric window in order to tune the window for maximum transmission at a frequency for which the window has a thickness smaller than a half-wavelength. The inductive screen is a metal or metal-coated sheet of a connected or disconnected loop structure, thereby allowing the generation of induced closed current loops inside the window. The operation of such a metal-dielectric window is based on the cancellation of the capacitive loading of the dielectric layer against the inductive loading of the conducting loops.
The second metal-dielectric window type incorporates a transparent Frequency Selective Surface (FSS) inside the window. The transparent FSS is a metal or metal-coated sheet with a periodic array of resonant slots cut in the metal surface. Such a window may include several dielectric layers and one or more FSSs. The operation of this metal-dielectric window is based on the resonance phenomena of the slots. The resonance frequencies strongly depend on the geometry of the slot, which may be rectangular, shaped like a cross, Jerusalem cross, square ring, circular ring, etc. In addition to the resonant slots, this window may include also a conductive mesh or conductive elements to block radiation of certain frequency bands, different from the transmission band. This is disclosed, for example, in U.S. Pat. No. 4,785,310, GB 2337860 and EP 096529.
Controllable windows enabling to tune the transmission band of the window have been developed, and are disclosed, for example, in U.S. Pat. No. 5,600,325. Such windows utilize ferroelectric materials capable of changing their dielectric constant in response to the application of DC voltage thereto. The main problem with these devices is associated with the supply of DC voltage without destroying the window transparency. According to the technique of U.S. Pat. No. 5,600,325, the FSS has complete electrical conductivity, and therefore DC voltage can be directly applied to the FSS.
All the basic window types as described above (i.e., utilizing a single half-wave dielectric layer, a single dielectric layer thinner than a half-wave and inductively loaded, and a single frequency selective surface) can generate only a single reflection zero within the operation frequency-band.